Most Coveted Endorsement for a Candidate? Ronald Reagan

He hasn't held office since 1989. In fact, he's been dead since 2004. But former President Ronald Reagan has been a regular presence on the campaign trail this year.

Republican presidential candidate Rick Santorum has made pilgrimages to Reagan's home town and the factory that makes his favorite candy as he tries to rally conservative voters to his fading campaign. Newt Gingrich regularly calls himself a "bold Reagan conservative." Mitt Romney has proposed a "Reagan economic zone" to boost free trade.

Even Democratic President Barack Obama has gotten in on the act, suggesting that the champion of the modern conservative movement would be viewed as too moderate in today's Republican party.

"Ronald Reagan, who, as I recall, is not accused of being a tax-and-spend socialist, understood repeatedly that when the deficit started to get out of control, that for him to make a deal he would have to propose both spending cuts and tax increases," Obama told the American Society of News Editors on Tuesday.

"I actually think Ronald Reagan would win handily in a primary and frankly in all the primaries," he told the same group on Wednesday.

Given Reagan's continued popularity with the public, Romney could be right. Though historians tend to rank him in the middle of the pack of U.S. presidents, recent public opinion polls have found that voters rank him No. 1 -- ahead of Abraham Lincoln and George Washington.

Reagan's popularity among Republicans isn't surprising, given the Republican presidents who followed.

George H.W. Bush lost his re-election bid in 1992 due to a recession and a tax-raising budget deal that angered conservatives. When his son, George W. Bush, left office in 2009, nearly three out of four voters said they disapproved of the way he had handled his job.

By contrast, Reagan's election in 1980 proved that conservative candidates could appeal to a broad spectrum of voters. The economy boomed as he cut taxes and deregulated, and the Soviet Union crumbled as he pursued an often-confrontational foreign policy. His approval rating was nearly as high when he stepped down as when he took the oath of office.

DEMOCRATS FIND HIM USEFUL, TOO

"Reagan took conservatism from a force of negative politics, and turned it into a force for positive governance," said Craig Shirley, a conservative media consultant who has written several books about Reagan.

Democrats are finding Reagan's legacy useful as well.

Obama's campaign would do well to study Reagan's 1984 re-election as it tries to find a message that will resonate as clearly as the "Morning in America" slogan that allowed an incumbent president to win a landslide victory in the face of high unemployment.

As they grapple with Republicans over fiscal issues, Democrats point out that a president best known for large income-tax cuts also signed off on numerous tax increases, agreed to raise the national debt limit 18 times, and crafted a compromise that put the Social Security retirement program on sound footing.

Obama has praised Reagan as he seeks to buff his bipartisan credentials.

"No matter what political disagreements you may have had with President Reagan - and I certainly had my share - there is no denying his leadership in the world, or his gift for communicating his vision for America" Obama wrote in USA Today in 2011.

As the bitter partisan disputes of the Reagan years recede into history, politicians of all stripes have an incentive to invoke his legacy, said Princeton University historian Julian Zelizer.

"When you have a president who's popular in the public imagination, that has a lot of value. Different people are going to take out of him what they can," Zelizer said.

Reagan's legacy has proven to be a tricky challenge for Romney, who was a registered independent in the 1980s and said he was "not trying to return to Reagan-Bush" in his unsuccessful bid for a Senate seat in liberal-leaning Massachusetts in 1994.

Romney supporters point out that Reagan's views evolved over time as well. Reagan was an early supporter of Democratic President Franklin Delano Roosevelt and, like Romney, came to regret his early support for abortion rights.

Romney has invoked Reagan less often on the campaign trail than his Republican rivals but has emphasized Reagan-like stances on foreign policy and regulation as he paints Obama as not assertive enough overseas and hostile to business.

He's likely to refer to the Gipper more often as the matchup against Obama heats up, Zelizer said.

"I assume when September and October roll around Romney will talk a lot about the Reagan legacy," he said.